More and more content is being delivered in digital form, and more and more digital content is being delivered online over private and public networks, such as Intranets and the Internet. For a client, digital form allows more sophisticated content, while online delivery improves timeliness and convenience. For a publisher, digital content also reduces delivery costs. Unfortunately, these worthwhile attributes are often outweighed in the minds of publishers by the corresponding disadvantage that online information delivery makes it relatively easy to obtain pristine digital content and to pirate the content at the expense and harm of the publisher.
Piracy of digital content, especially online digital content, is not yet a great problem. Most premium content that is available on the Web is of low value, and therefore casual and organized pirates do not yet see an attractive business stealing and reselling content. Increasingly, though, higher-value content is becoming available. Books and audio recordings are available now, and as bandwidths increase, video content will start to appear. With the increase in value of online digital content, the attractiveness of organized and casual theft increases.
The unusual property of digital content is that the publisher (or reseller) gives or sells the content to a client, but continues to restrict rights to use the content even after the content is under the sole physical control of the client. For instance, a publisher will typically retain copyright to a work so that the client cannot reproduce or publish the work without permission. A publisher could also adjust pricing according to whether the client is allowed to make a persistent copy, or is just allowed to view the content online as it is delivered. These scenarios reveal a peculiar arrangement. The user that possesses the digital bits often does not have full rights to their use; instead, the provider retains at least some of the rights. In a very real sense, the legitimate user of a computer can be an adversary of the data or content provider.
“Digital rights management” is therefore fast becoming a central requirement if online commerce is to continue its rapid growth. Content providers and the computer industry must quickly address technologies and protocol for ensuring that digital content is properly handled in accordance with the rights granted by the publisher. If measures are not taken, traditional content providers may be put out of business by widespread theft, or, more likely, will refuse altogether to deliver content online.
Traditional security systems ill serve this problem. There are highly secure schemes for encrypting data on networks, authenticating users, revoking certificates, and storing data securely. Unfortunately, none of these systems address the assurance of content security after it has been delivered to a client's machine. Traditional uses of smart cards offer little help. Smart cards merely provide authentication, storage, and encryption capabilities. Ultimately, useful content must be assembled within the host machine for display, and again, at this point the bits are subject to theft. Cryptographic coprocessors provide higher-performance cryptographic operations, and are usually programmable but again, fundamentally, any operating system or sufficiently privileged application, trusted or not, can use the services of the cryptographic processor.
There appear to be three solutions to this problem. One solution is to do away with general-purpose computing devices and use special-purpose tamper-resistant boxes for delivery, storage, and display of secure content. This is the approach adopted by the cable industry and their set-top boxes, and looks set to be the model for DVD-video presentation. The second solution is to use secret, proprietary data formats and applications software, or to use tamper-resistant software containers, in the hope that the resulting complexity will substantially impede piracy. The third solution is to modify the general-purpose computer to support a general model of client-side content security and digital rights management.
A fundamental building block for client-side content security is a secure operating system. If a computer can be booted only into an operating system that itself honors content rights, and allows only compliant applications to access rights-restricted data, then data integrity within the machine can be assured. This stepping-stone to a secure operating system is sometimes called “Secure Boot.” If secure boot cannot be assured, then whatever rights management system the secure OS provides, the computer can always be booted into an insecure operating system as a step to compromise it.
Secure boot of an operating system is usually a multi-stage process. A securely booted computer runs a trusted program at startup. The trusted program loads an initial layer of the operating system and checks its integrity (by using a code signature or by other means) before allowing it to run. This layer will in turn load and check the succeeding layers. This proceeds all the way to loading trusted (signed) device drivers, and finally the trusted application(s).
An article by B. Lampson, M. Abadi, and M. Burrows, entitled “Authentication in Distributed Systems: Theory and Practice,” ACM Transactions on Computer Systems v10, 265, 1992, describes in general terms the requirements for securely booting an operating system. The only hardware assist is a register that holds a machine secret. When boot begins this register becomes readable, and there's a hardware operation to make this secret unreadable. Once it's unreadable, it stays unreadable until the next boot. The boot code mints a public-key pair and a certificate that the operating system can use to authenticate itself to other parties in order to establish trust.
Clark and Hoffman's BITS system is designed to support secure boot from a smart card. P. C. Clark and L. J. Hoffman, “BITS: A Smartcard Operating System,” Comm. ACM. 37, 66, 1994. In their design, the smart card holds the boot sector, and PCs are designed to boot from the smart card. The smart card continues to be involved in the boot process (for example, the smart card holds the signatures or keys of other parts of the OS).
Bennet Yee describes a scheme in which a secure processor first gets control of the booting machine. B. Yee, “Using Secure Coprocessors”, Ph.D. Thesis, Carnegie Mellon University, 1994. The secure processor can check code integrity before loading other systems. One of the nice features of this scheme is that there is a tamper-resistant device that can later be queried for the details of the running operating system.
Another secure boot model, known as AEGIS®, is disclosed by W. Arbaugh, D. G. Farber, and J. M Smith in a paper entitled “A Secure and Reliable Bootstrap Architecture”, Univ. of Penn. Dept. of CIS Technical Report, IEEE Symposium on Security and Privacy, page 65, 1997. This AEGIS® model requires a tamper-resistant BIOS that has hard-wired into it the signature of the following stage. This scheme has the very considerable advantage that it works well with current microprocessors and the current PC architecture, but has three drawbacks. First, the set of trusted operating systems or trusted publishers must be wired into the BIOS. Second, if the content is valuable enough (for instance, e-cash or Hollywood videos), users will find a way of replacing the BIOS with one that permits an insecure boot. Third, when obtaining data from a network server, the client has no way of proving to the remote server that it is indeed running a trusted system.
On the more general subject of client-side rights management, several systems exist or have been proposed to encapsulate data and rights in a tamper-resistant software package. An early example is IBM®'s Cryptolope®. Another existent commercial implementation of a rights management system has been developed by Intertrust. In the audio domain, AT&T® Research have proposed their “A2b®” audio rights management system based on the PolicyMaker rights management system.